SEATTLE — For the last three years, not one calf has been born to the dwindling pods of black-and-white killer whales spouting geysers of mist off the coast in the Pacific Northwest.

Normally four or five calves would be born each year among this fairly unusual urban population of whales — pods named J, K and L. But most recently, the number of orcas here has dwindled to just 75, a 30-year low in what seems to be an inexorable, perplexing decline.

Listed as endangered since 2005, the orcas are essentially starving, as their primary prey, the Chinook, or king salmon, are dying off. Just last month, another one of the Southern Resident killer whales — one nicknamed “Crewser” that hadn’t been seen since last November — was presumed dead by the Center for Whale Research.

In March, Gov. Jay Inslee issued an executive order directing state agencies to do more to protect the whales, and in May he convened the Southern Resident Orca Task Force, a group of state, tribal, provincial and federal officials, to devise ways to stem the loss of the beloved regional creature. “I believe we have orcas in our soul in this state,” he said. At another point, he wrote of the whales and Chinook salmon that “the impacts of letting these two species disappear would be felt for generations.”

The orcas are also facing a new threat. The recent agreement between the Canadian government and Kinder Morgan to expand the Trans Mountain Pipeline would multiply oil tanker traffic through the orcas’ habitat by seven times, according to some estimates, and expose them to excessive noise and potential spills. Construction is set to begin in August, despite opposition from Governor Inslee and many environmentalists.

In the late 1990s, there were nearly 100 of these giant whales in the population. Following the salmon, they migrate in the Salish Sea to the northern coast of British Columbia and often surface in the south at Puget Sound within sight of downtown Seattle, especially during the spring and summer months. The males, which can weigh up to 22,000 pounds, typically live about 30 years, and females, up to 16,000 pounds, survive longer — up to 50 or 60 years, although one J-pod member, Granny, lived to be 105 years old.

Not only are there fewer calves in recent years, but signs of inbreeding also point to a weakening population. In the 1970s and 80s, theme parks like Sea World captured nearly 4 dozen orcas from the region, possibly shrinking the pods’ gene pool. In the last three decades, just two males fathered half the calves in the last three decades, and only a third of the females are breeding, just once every decade instead of every five years. Researchers worry that reproducing females are aging out of the population, and won’t be replaced.

Some conservationists are concerned that the orcas’ decline is another sign of a marine ecosystem in collapse. Beginning in 2013, something known as “The Blob” — a gigantic mass of nutrient poor, extremely warm water — warmed the Pacific from Mexico to Alaska, as much as six degrees above normal. Several years ago, starfish succumbed to a wasting disease and vanished from tide pools.

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Scientists suspect the biggest contributing factor endangering the orcas may be the disappearance of Chinook salmon, which is also endangered. The whales eat 30 a day, and hunting enough smaller prey requires a lot more energy.CreditSteve Martarano/U.S. Fish and Wildlife Service, via Associated Press

Much is still unknown about the plight of these orcas, but biologists and conservation managers have zeroed in on several main factors — and they are all connected.

The biggest contributing factor may be the disappearance of big king salmon — fish more than 40 inches long. “They are Chinook salmon specialists,” said Brad Hanson, team leader for research at the Northwest Fisheries Science Center here, part of NOAA. “If they could, they would eat Chinook salmon 24/7.” Orcas gobble 30 a day. Hunting enough smaller prey requires a lot more energy.

The underwater world in the region is also getting noisier, especially an area between the San Juan Islands and Vancouver Island called Haro Strait. It is one of the orcas’ favorite foraging grounds in the summer.

In recent years, officials have expanded the distance which vessels, including whale watching boats and kayaks, must keep from the whales. And there is a voluntary no-go zone near the San Juan Islands.

“Just the presence of boats can cause the whales to spend less time feeding,” said Lynne Barre, of NOAA Fisheries, recovery coordinator for the orcas. “And it’s harder to communicate. They have to call longer and louder when boats are nearby.”

Another factor is the pollution in Puget Sound. Whales that live off the coast of Seattle, Tacoma and other cities are effectively urban whales buffeted by municipal and industrial waste, and the occasional spillage from wastewater treatment plants into the ocean. Killer whales carry some of the highest levels of pollution of any marine animal.

Of most concern are the lingering effects of chemicals and pesticides, including the now banned DDT, as well as PCBs and PBDE, widely used in flame retardants and found through the world. The pollutants accumulate in salmon as they feed, and when the whales eat salmon they also ingest PCBs at even higher levels.

“It’s very lipophilic, which means it stays in the fat, and the females transfer a huge proportion of the contaminant burden to their offspring,” Dr. Hanson said. “About 85 percent gets transferred to calves through lactation.”

And while much of the pollution is from the region’s industrial past, Boeing disclosed this spring that over the past five years it had discharged highly toxic PCBs into the Duwamish River, which flows into Puget Sound, thousands of times over the legal limit.

These toxins suppress the whales’ immune systems, making them more susceptible to disease. They can also impede reproduction. That may be why tests show a high number of females who have become pregnant have failed to calve.

However, the decline of the whales can’t be pegged, experts say, to contaminants alone. A separate population of transient whales near here eat mammals that eat fish, and so consume concentrate contaminants at even higher levels — many times as high as the resident pods. Yet they are thriving, which has left scientists scratching their heads. Global populations are robust as well.

One possible scenario is that the dearth of salmon coupled with the interference of engine noise, which can affect their immune system, too, deprives the orcas of a sufficient diet. Their bodies then draw on fat reserves, which are laced with chemicals that suppress their immune system and reduce fecundity.

But experts aren’t sure what is raising their mortality rate. Often, when whales die, their carcasses sink or wash up onto remote beaches and are hard to find and test.

In recent years, researchers have been focusing on anthroponeses, diseases that humans may be passing to wildlife. Scientists have sailed out among the pods with a petri dish at the end of a 25-foot long pole to pass through the mist that whales exhale and see what they carry in their lungs. They found a range of pathogens that could be from humans, including antibiotic resistant bacteria and staphylococcus, which can cause pneumonia.

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Orcas swimming within view of downtown Seattle was a far more common sight decades ago.CreditJoel Rogers, via Getty Images

“It doesn’t mean they are sick, we don’t have evidence for that,” said Linda D. Rhodes, a research biologist expert in marine microbes and toxins and part of the study. “It means they are being exposed. Whether or not the whales get sick is a product of how much of it is present in the environment and how well is the whale able to defend itself.”

There is deep concern that a fatal human or animal disease has, or will, cross the species barrier and find its way into these immuno-compromised killer whales. “I’ve had dreams about it at night,” said Joseph K. Gaydos, a veterinarian with the SeaDoc Society in Eastsound, Washington in the San Juan Islands, who studies the southern residents. “Disease smolders in the environment but can break out. If there were a highly virulent virus to come through here it would take out a large part of the population and totally stop recovery efforts.”

Disease threats are myriad. A young killer whale died from a fungal infection last year. Toxoplasmosis is a disease spread by parasites in the feces of cats. It is one of the top threats to the Hawaiian monk seal, killing eight of the remaining 1,400 since 2001. It’s not known, though, to affect whales.

Canine distemper from dogs is also a concern. It’s a morbillivirus, which is an RNA rather than a DNA virus. Some 1,500 dolphins were killed by a single outbreak of morbilliviruses on the East Coast several years ago.

Steps are being planned to help the whales persevere. More Chinook salmon are being reared in hatcheries as whale food, but that is far from a certain fix.

In the end trying to maintain a population of whales in the shadow of one of the fastest growing cities in the country may not be possible.

“It’s an ecosystem-wide problem,” Dr. Hanson said. “Things are out of whack and we have to get them back to where we can sustain killer whales. And the clock is ticking.”

Losing the charismatic, intelligent animals with the distinctive black-and-white “paint job” and permanent smile would be a blow to the area.

“There would be a great sense of loss,” Dr. Rhodes said. “They are such a part of our identity here. It would be a real sense of failure.”

Correction:July 10, 2018

An earlier version of this article misstated Brad Hanson’s position at the Northwest Fisheries Science Center. He is the team leader for research, not recovery efforts. The article also misstated the acronym for polybrominated diphenyl ethers, widely used in fire retardants. It is PBDE, not PPDE.